A NOR gate is a semiconductor integrated logic circuit that has a high output when all the inputs are logic low and a low output when at least one or more inputs are logic high. The number of inputs may range from two to twenty in a typical integrated circuit. Each input is typically a gate terminal for one of a plurality of pull-down n-channel CMOS (complementary metal-oxide semiconductor) transistors arranged in parallel and sourced by a single pullup transistor.
Inasmuch as NOR gates are frequently used in the critical delay path where speed is a primary concern, a great deal of effort has been expended over the years to reduce the propagation delay of NOR gates, viz., the time delay between the appearance of an input signal at the output.
Unfortunately, several problems plague the conventional multiple input NOR gate. First, the low level of the output signal varies depending on the number of input transistors that are turned on. The more the number of transistors are turned on, the lower the output voltage. Since the logic high output voltage is almost always at the source level (V.sub.cc), the voltage swing on the output could be from zero to V.sub.cc or rail-to-rail.
There are shortcomings associated with this design. First, due to capacitive effects, the rail-to-rail voltage swing results in poor propagation delay. Second, the low-to-high transition on the output is poor at high operating temperatures because the current conduction of the pull-up transistor degrades with temperature due to a decrease in carrier mobility. Consequently, the current charging the output node decreases with temperature, resulting in slower propagation delay on the low-to-high transition.
These prior NOR gate designs do not provide adequate speed for some current high-speed integrated circuit applications.
Thus, there is a long felt and unsatisfied need for an improved multiple input NOR gate design. Specifically, the multiple input NOR gate design should be one that offers high speed performance due to a small output voltage swings and good low-to-high transition at the output without speed degradation at high temperatures.